1. Field of the Invention
The invention relates to a method for producing an article by reactive infiltration and to an article so produced.
2. Description of the Prior Art
In processes of forming metal-based composite materials, also known as metal matrix composites, a preform composed of a solid reinforcement material is often infiltrated with a liquid metal, which after solidification forms a solid matrix surrounding the reinforcement material. A chemical reaction or reactions between the infiltrating liquid metal and the solid reinforcement are often detrimental and are usually avoided. Further, the liquid infiltrant often does not wet the preform and pressure must be applied in order to force the infiltrant into the porous preform. For example, the processing of aluminum-carbon and aluminum-silicon carbide composites typically requires the use of pressure infiltration techniques such as pressure-casting or squeeze-casting, since aluminum does not wet either carbon or silicon carbide. Pressure infiltration processing is costly and, furthermore, imposes constraints on the shape of the preform to be infiltrated and, thus, upon the shape of the finished article to be produced. For example, shapes including re-entrant surfaces are difficult to process using a pressure-infiltration technique where a mold is required.
In a reactive infiltration process, a compound is formed by infiltration of a porous, solid preform with a liquid which chemically reacts with the preform to form a desired compound. By contrast with metal matrix composite processing by infiltration, as previously described, in a reactive infiltration process, chemical reaction between preform and infiltrant is desired, and is, in fact, promoted. The preform material and infiltrating liquid must be chemically reactive, and are chosen based on the compound which is to be formed. In reactive infiltration, the liquid may not infiltrate the porous solid preform without applied pressure, rendering processing difficult. In a reactive infiltration process, it is thus desirable to have both a liquid which will infiltrate the solid preform without applied pressure as well as be chemically reactive with the preform to form the desired compound.
Pressureless infiltration techniques have been developed, such as that described in U.S. Pat. No. 4,904,446 to White et al., issued Feb. 27, 1990. According to the White et al. method, the characteristics of the boron nitride preform, including preform porosity, are controlled so that infiltrant metal is drawn or transported through the preform by wicking or capillary action.
Thus there exists a need for a near-net shape, relatively economical pressureless infiltration process wherein spontaneous preform infiltration is accomplished by controlling liquid infiltrant composition so that the liquid infiltrant wets the preform.